The WCMM will prefer cooler water than the betta. Platies or guppies will work depending on the betta's personality. Some bettas do not tolerate long finned fish very well which would eliminate a nice male guppy from consideration.
The WCMMs would go fine with the C. paleatus (peppered corys) because they like the cooler water that the corys prefer. It's the betta what would be uncomfortable.